POROUS TIP 



Wing tip porosity refers to perforations, of varying size, distribu- 

 tion, and density, which connect the pressure and suction sides of the 

 lifting surface tip. The porosity concept may produce several beneficial 

 effects with regard to delay of tip vortex cavitation, e.g., the perfora- 

 tions connecting the pressure and suction sides will tend to decrease the 

 local circulation and shift the spanwise load distribution inboard, away 

 from the tip, thereby reducing the tip vortex strength. In addition, the 

 perforations may produce a higher level of turbulent flow entering the 

 vortex core and also direct an additional opposing mass flow into the core, 

 both of which would tend to produce destabilizing effects. 



As indicated in Table 1, this concept has received much attention for 



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aircraft application. Investigations, both model and full-scale, have 



indicated a substantial reduction in the vortex core near-field tangential 



velocity, with a minimal drag penalty. 



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 Figure 8 presents wind tunnel data which shows the effect of wing 



tip porosity on both the tip vortex characteristics and the wing perfor- 

 mance. As seen for a modest tip porosity of approximately 15 percent, a 

 substantial reduction of up to 70 percent in the vortex core tangential 

 velocity is achieved for a minimal decrease and increase of the wing 

 efficiency and drag, respectively. 



Similar qualitative results are also reported for a porous tipped 

 propeller. However, careful attention must be given to the perforation 

 alignment and finishing in order to avoid local surface cavitation 



For the alleviation of tip vortex cavitation on marine propellers, the 

 porosity concept appears to be attractive. The results indicate that it is 

 efficient and it is extremely simple and practical. However, care must be 

 exercised to avoid local surface cavitation. 



ENDPLATES, WINGLETS, FENCES, AND CONTRAVANES 



The attachment of vertical endplates to the wing tip (Figure 4) signif- 

 icantly interferes with the tip vortex rollup. Numerous investigations re- 

 port that the endplate retards the rollup, thereby increasing the tip 

 loading. In addition, the increased endplate surface area tends to disperse 



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